Protein kinases are a group of enzymes that regulate the activity of their target proteins by the addition of phosphate groups to the protein substrate. Kinases play an essential role in many physiological processes including cell division, differentiation, cellular homeostasis and signal transduction. Kinases can be subdivided by their target into Serine/Threonine kinases and Tyrosine kinases. Tyrosine kinases are further subdivided into receptor tyrosine kinases and non-receptor tyrosine kinases. The mammalian Janus kinase (JAK) family members are non-receptor tyrosine kinases.
The JAK family has four members; JAK1, JAK2, JAK3 and TYK2. JAK1, JAK2 and TYK2 are universally expressed, whereas JAK3 expression is limited to hematopoetic cells. The JAK family is involved in intracellular signal transduction from >70 different cytokines. Cytokines bind to their cell surface receptors resulting in receptor dimerization and subsequent activation/phosphorylation of JAK tyrosine kinases. The JAKs are either constitutively associated with the receptor or are recruited upon cytokine binding. Specific tyrosine residues on the receptor are then phosphorylated by activated JAKs and serve as docking sites for STAT proteins. STATs are phosphorylated by JAKs, dimerize, then translocate to the nucleus where they bind specific DNA elements and activate gene transcription. JAK1 signals in conjunction with all JAK isoforms in a cytokine dependent manner.
A considerable body of literature has accumulated that link the JAK/STAT pathway to various diseases and disorders including hyperproliferative disorders and cancer such as leukemia and lymphomas, immunological and inflammatory disorders such as transplant rejection, asthma, chronic obstructive pulmonary disease, allergies, rheumatoid arthritis, type I diabetes, amyotropic lateral sclerosis and multiple sclerosis.
Results from clinical trials with multi JAK inhibitors tofacitinib citrate (Xeljanz®, Pfizer) a JAK1/JAK2/JAK3 inhibitor and baricitinib (Lilly) a selective JAK1 and JAK2 inhibitor support the hypothesis that high levels of efficacy can be achieved through targeting JAK inhibition. (For tofacitinib studies see: Kremer et al., Arthritis Rheum. (2012) 64(5): 1487, “The safety and efficacy of a JAK inhibitor in patients with active rheumatoid arthritis”; Fleishmann R. et al., Int J. Rheum Dis. 2016 Jul. 24, “Tofacitinib in patients with active rheumatoid arthritis”; Fleischmann R. et al. NEJM (2012) 367:495-507, “Placebo controlled trial of tofacitinib monotherapy in rheumatoid arthritis”; and for baricitinib see, Greenwald, M. W. et al.; Arthritis & Rheum., Vol. 62, November 2010 Abst. Suppl. “A randomized dose-ranging, PBO-controlled study of TNCB028050, a selective JAK1 and JAK2 inhibitor, in subjects with active rheumatoid arthritis”, respectively). However, dose limiting adverse events (AEs) have limited the efficacy and use of these agents. Significant hematopoietic AEs, specifically anemia, were observed in patients taking both tofacitinib and baricitinib, with a greater incidence and severity at higher doses. (See Yamaoka, K., Current Opinion Chem. Bio. (2016) 32:29-33; and Greenwald, M. W. et al.; Arthritis & Rheum., Vol. 62, November 2010 Abst. Suppl.).
The occurrence of anemia is believed to be due to inhibition of erythropoietin (EPO) signaling. EPO is a growth factor critical for red blood cell development that signals via JAK2. Inhibition of EPO also leads to an inability to recover from anemia of chronic disease. Approximately 40% of rheumatoid arthritis (RA) patients suffer from anemia of chronic disease (See, Masson, C. et al., Joint Bone Spine 78(2011):131-137, “Rheumatopid Anemia”; and Han et al., J. Rheumatol. (2007)34 (11):2177-2182), “Association of anemia and physical disability among patients with rheumatoid arthritis”. The current treatment paradigm is to treat the inflammation that causes this anemia; however treatment with multi-JAK inhibitors that also inhibit EPO signaling cancels out the benefits on hemoglobin levels from treating the inflammation. Specific JAK1 inhibitors would not impact EPO signaling, would not be limited by anemia AEs, and would allow hemoglobin levels to rebound after inflammation was reversed.
Additional clinical evidence supporting the JAK1 hypothesis comes from tocilizumab (Actemra®, Hoffmann-La Roche), a humanized monoclonal antibody against the interleukin-6 (IL-6) receptor. IL-6 signals through JAK1 and JAK2 pathway. High levels of efficacy are achieved with this biologic agent without inducing anemia, and anemia of inflammation is successfully reversed (See Emery, P., et al., Ann. Rheum. Dis. (2008) 67:1516-1523, “IL-6 receptor inhibition with tocilizumab improves treatment outcomes in patients with RA refractory to anti-TNF biologicals: results of a 24-week multicenter randomized placebo-controlled trial”; and Hashizume, M. et al., Rheumatol. Int. (2010) 30(7):917-23), “Tocilizumab, a humanized anti-IL-6 receptor antibody, improved anemia in monkey arthritis by suppressing IL-6 induced hepcidin production”.
There is a need to identify JAK1 specific inhibitors that do not impact EPO signaling in order to mitigate the occurrence of anemia adverse events.
WO 2013/041042 discloses pyrazole carboxamdines as Janus Kinase Inhibitors that are useful for the treatment of rheumatoid arthritis, asthma, COPD and cancer. WO 2013/041042 discloses compounds of the following formula:
